The present invention relates to modified silicon dioxide particles, and also to a process for preparing modified silicon dioxide particles and to the products obtainable by that process, to particular end uses of the silicon dioxide particles, and to nanocomposites which comprise the silicon dioxide particles. Also proposed are silica sols comprising the modified silicon dioxide particles.
Silica sol is a suspension of amorphous silicon dioxide (SiO2) in which the silicon dioxide is present in the form of spherical individual particles which are substantially (i.e., to an extent of at least 50%, preferably to an extent of at least 70%, 80% or 90%) noncrosslinked with one another. The dispersion medium may be different—thus, for example, a solvent or a monomer.
Silica sols are widely used. For example, they are suitable for use as binders for precision casting, for fibers in the refractories sector, and in the production of catalysts, as coating agents for films (antiblocking), in the textile segment for nonslip finishes, in the construction sector as additives for air-placed concrete, or as binders for fire protection and thermal insulation applications, as polishing agents for electronics or else in the paper segment, in the context, for example, of paper retention or as an additive in the coating of speciality papers.
Conventional silica sols, depending on the size of the silicon dioxide particles, are milkily cloudy through opalescent to colorlessly clear in form. The particles generally have diameters of 5 nm to 150 nm and are usually spherical, three-dimensionally limited, and preferably negatively charged electrically. In the interior of the individual particles there is typically a framework of siloxane compounds which is a result of the linking of [SiO4] tetrahedra and/or of polysilicic acids.
In view of their small size, the particles have a large specific surface area, leading in turn to a high surface energy. One unwanted consequence of this high surface energy is that the particles tend to form agglomerates or even aggregates. Forming stable dispersions comprising silicon dioxide particles is therefore not readily possible.
In order as far as possible to prevent the agglomeration or aggregation of the particles, it is known from the prior art to modify them on their surface.
Thus “The Chemistry of Silica” by Ralph K. ller (1979. John Wiley & Sons, Inc.; New York, Chichester, Brisbane, Toronto) discloses modifying silicate surfaces with chlorosilanes.
EP 0 982 268 A describes the reaction of aqueous silica sols in the presence of water-miscible solvents with halosilanes and mixtures of siloxanes and halosilanes and/or siloxanes.
U.S. Pat. No. 6,736,891 describes the reaction of an aqueous suspension of precipitated silicas at a low pH with hexamethyldisiloxane in the presence of isopropanol.
The flexibility of the modified silicon dioxide particles and their preparation processes, known from the prior art, is inadequate. For instance, the possibilities of adapting the particles for further processing to composites are limited.
U.S. Pat. No. 2,801,185 discloses an organic, surface-modified silicon dioxide particle and also a process for preparing it, in which a water-comprising silica sol is admixed with an organic solvent and the water is removed azeotropically, the water fraction thus being lowered to below 1%. Subsequently a modifier (coating material) is added and the modification of the surface is performed. Coating materials mentioned include saturated primary and secondary alcohols. A disadvantage of these particles, however, is that they do not have the desired stability and/or in some cases do not allow the desired flexibility in further processing.
From U.S. Pat. No. 2,786,042 it is known to modify organic silica sols with hydrocarbon-containing silanols on the surface.
A disadvantage of these known processes, accordingly, is that they result in modified particles of only limited redispersibility. Furthermore, their compatibility with organic solvents, such as toluene and hexane, or with organic resins and polymers, is restricted. Furthermore, the particles can be modified only within narrow limits, and so a flexible adaptation is not possible.